The present invention relates to fluked burial devices adapted for burying into a soil and more particularly to marine anchors, cable depressors and such-like fluked devices adapted for burying into submerged soil.
A marine anchor comprising a shank with a cable attachment point at the forward end and a fluke structure attached thereto has a fluke angle .theta. defined by the angle between the fore-and-aft central line of the fluke structure and the line from the said cable attachment point to the rear of the fluke structure measured in the vertical plane of symmetry. Up until now, this angle .theta. has been in the range 28.degree. to 50.degree. with the anchor embedded in the soil. Fluke angles in the range 28.degree. to 35.degree. have generally been found to give optimum anchor performance in granular non-cohesive soils such as sand and gravel, since this relatively low fluke angle enables the anchor fluke more readily to penetrate the firmer soils formed of sand or gravel. On the other hand a fluke angle of approximately 50.degree. has been found necessary to give optimum performance in cohesive soils such as soft clay and mud. This is due to the fact that in such cohesive soils as mud, the forward end of the shank of the anchor tends to tilt upwardly when the anchor is in the fully buried condition thereby seriously reducing the actual or effective angle of attack of the fluke. Provision of the relatively high fluke angle of 50.degree. enables this operational disadvantage to be substantially overcome and satisfactory anchor holding force maintained.
For ship use, anchors usually have a fluke angle in the region of 40.degree. to provide a reasonable compromise performance when used in either non-cohesive or cohesive soils. For offshore drilling vessels or pipelaying barges using multiple anchor spread moorings, anchors generally have means for adjusting the fluke angle to give optimum performance according to the soil type in which the anchors are deployed. Unfortunately, the nature of the mooring bed soil often is unknown prior to deploying anchors and several anchors may be deployed before it is realised that incorrect fluke angles have been selected. These anchors must then be retrieved for fluke angle adjustment and redeployed. This wastes time and consequently incurs high costs.
It is an object of the present invention to obviate or mitigate these disadvantages.
According to one aspect of the present invention there is provided a fluke burial device, particularly an anchor having a burial fluke member orientated to provide a positive burial angle for digging into a bed of soil when the burial device is in the vertical working burial attitude, a cable attachment member attached to said fluke member, soil barrier means located substantially above the burial fluke member when the burial device is in said vertical working burial attitude such that a straight line from a foremost extremity of the fluke member to an upper edge of the soil barrier means lies in the range 8.degree. to 24.degree. to the upper surface of the fluke member, a major portion of the soil barrier means being located aft of the rear edge of the burial fluke member and such that the rear of the soil barrier means has a horizontal separation from the rear of the burial fluke member not more than half the overall longitudinal length of the fluke member, said soil barrier means including at least one soil barrier surface which is inclined relative to said fluke member, said soil barrier surface having an area less than the upper surface area of the fluke member, and passage means associated with said soil barrier means to permit escape of non-cohesive soil passing over the fluke member.
According to another aspect of the present invention there is provided a fluke burial device, particularly an anchor having a burial fluke member orientated to provide a positive burial angle for digging into a bed of soil when the burial device is in the vertical working burial altitude, a cable attachment member attached to said fluke member, soil barrier means located substantially above the burial fluke member when the burial device is in said vertical working burial altitude, the major portion of the soil barrier means being located aft of the rear edge of the burial fluke member and such that the rear of the soil barrier means has a horizontal separation from the rear of the burial fluke member not more than half the overall longitudinal length of the fluke member, said soil barrier means including at least one soil barrier surface which is inclined with a forwardly opening acute angle relative to said burial fluke member, said soil barrier surface having an area less than the upper surface area of the fluke member, and soil passage means associated with said soil barrier means to permit escape of non-cohesive soil passing over the fluke member.